1. Field of the Invention
The present invention relates to thermal control of heat-generating devices within a system, and more particularly to powering the means used to cool heat-generating devices within a downhole tool string.
2. Background of the Related Art
The drilling of oil and gas wells (also known as wellbores or boreholes) typically involves the use of several different measurement and telemetry systems to acquire data representing certain parameters of a subsurface formation (also known as a downhole reservoir) via a borehole penetrating the formation, and data representing the state of various drilling and/or borehole survey parameters. In measurement-while-drilling (MWD) and logging-while-drilling (LWD) tools, data is typically acquired by sensors located in the drill string near the drill bit. This data is either stored in downhole memory or transmitted to the surface using a telemetry means, such as mud flow telemetry devices.
Both the downhole sensors and the telemetry means of the MWD/LWD tools require electrical power. Since it is not feasible to run a power supply cable from the surface through the drill string to the sensors or the telemetry means, electrical power must be obtained downhole. The state of the art MWD and LWD devices obtain electrical power downhole from energy sources such as a turbine-based alternator, a battery pack, or combinations thereof. Examples of alternators used in downhole tools are shown in U.S. Pat. No. 5,517,464, assigned to the assignee of the present invention, and U.S. Pat. No. 5,793,625 assigned to Baker Hughes. Because of the physics involved, as well as practical design limitations, these types of energy sources (as well as others) often generate excess quantities of energy that the downhole tools cannot actively consume. Such excess energy must therefore be dissipated as waste energy through an energy sink, such as by transferring heat through the wall of a drill collar to the drilling fluid (also known as mud) being circulated through the borehole annulus.
It will be appreciated by those having ordinary skill in the art that virtually all downhole tools generate appreciable quantities of heat. Thus, e.g., operating LWD tools may typically generate heat on the order of 10 W or more; operating MWD tools may typically generate heat on the order of 100 W or more; and mechanical tools such as direction drilling tools may generate heat on the order of 1 KW or more when actuated. Sustained heat exposure can detrimentally affect the performance of these tools. Moreover, these tools often include electronic components such as controllers, amplifiers, oscillators, filters, switches, clocks, etc., that are very sensitive to temperatures above their rated or specified levels, or to sustained exposure at high temperature, particularly when exposed to temperatures above their maximum rating or specifications. Overheating of electronic components can result in improper operation, reduced accuracy, immediate failure, or a significant reduction in life. However, the ambient temperature of the borehole environment at depth is often elevated to the point that the mud circulating therethrough may not provide adequate cooling for the downhole tools and their respective components. A need therefore exists for providing supplemental cooling in downhole tool strings.
At least one known publication, U.S. Pat. No. 5,931,000 issued to Turner et al, describes the use of thermoelectric coolers (“TECs”) for providing supplemental cooling of electronic components within downhole tools. However, the current required to effectively operate one or more TECs is very high, on the order of several Amps—amounts that are not always readily available or efficiently generated by a down-hole tool string. One aspect of the present invention recognizes that available waste energy, particularly in significant amounts, can be used to advantage with TECs. The Turner patent fails to disclose that excess energy may be diverted (i.e., recycled) to energize the TECs. Accordingly, a need exists for applying the excess energy that would otherwise be dissipated to downhole tool strings in such a manner that the requisite supplemental cooling is achieved.